The invention relates to a particle-optical apparatus for irradiating an object by means of a beam of electrically charged particles,
which apparatus is provided with an object carrier for carrying the object to be irradiated,
which object carrier is provided with a system of support elements for supporting the object, which system of support elements defines a substantially flat support surface,
the support elements being resiliently movable in a direction transversely of the support surface.
U.S. Pat. No. 5,605,574 discloses an apparatus for the treatment of wafers for integrated semiconductor circuits. In this known apparatus an object, being the semiconductor wafer, is carried by an object carrier which is provided with a number of support elements for supporting the wafer being treated in the apparatus. For uniform support of the wafer the support points of the support elements together define a substantially flat support surface. The wafers to be treated, however, are not completely flat and, moreover, the shape error of individual wafers varies highly. In order to make all support elements participate in supporting the wafer to a more or less equal degree despite the shape errors, they are constructed so as to be elastic so that they can accommodate to the shape errors even when the flatness of the wafer changes during the treatment.
It may be desirable to irradiate the wafers being treated during the production of integrated circuits by means of a beam of electrically charged particles such as electrons or ions. Such irradiation may be aimed at inspection or treatment of the wafers, for example, the repair of defects in the integrated circuit by means of an ion beam. This usually requires a resolution of the order of magnitude of one percent of the smallest detail size of the integrated circuits; considering the contemporary of detail size this means a resolution of approximately 3 nm.
Vibrations are liable to occur in the wafers which are elastically supported in the known manner (which wafers have a diameter of 30 cm according to the present state of the art), the vibration direction extending mainly perpendicularly to the surface of the wafer. Such vibrations are caused, for example by acoustic vibrations in the vicinity of the particle-optical apparatus (for example, an electron microscope) which are transferred to the apparatus via the ambient atmosphere, or by building vibrations which penetrate as residual vibrations despite the resilient and damped mounting of such an apparatus. The vibrations are transferred to the wafers in given points (for example, near fixed points); other regions of the wafer may then exhibit a substantially higher vibration deviation due to resonance. The amplitude of such vibrations due to resonance is not reduced because, since the wafers are situated in vacuum in such apparatus, no damping effect is exerted by an ambient atmosphere. Consequently, such vibrations may reach an amplitude which is much higher than that encountered in comparable situations in atmospheric circumstances.
When the axis of the irradiating ion beam or electron beam extends perpendicularly to the vibrating surface, such a vibration will cause, generally speaking, only a slight loss of resolution. For the inspection of wafers, however, it is often necessary to irradiate the wafer in a tilted position relative to the beam, the angle of tilt then amounting to as much as 60xc2x0; this means that the angle between the beam axis and the wafer surface amounts to 30xc2x0. The transverse displacement then caused relative to the beam by the vibration of the surface region to be observed is of the same order of magnitude as the vibration amplitude. Such a transverse displacement makes it impossible to achieve said required resolution of 3 nm.
It is an object of the invention to provide a particle-optical apparatus of the kind set forth wherein the amplitude of said wafer vibrations is substantially lower than in the case of the known elastic support. To this end, the apparatus according to the invention is characterized in that the support elements which are resiliently movable transversely of the support surface are in frictional contact with the object carrier. The invention is based on the recognition of the fact that the combination of two steps is necessary so as to counteract the vibrations which are detrimental to the desired resolution. The first step of this combination consists in ensuring that all of the support points of the support elements contact the wafer to be supported, irrespective of the shape errors of the individual wafers relative to the ideal flat surface. This is achieved in that the support elements are resiliently movable in the direction transversely of the support surface. The second step of the combination consists in ensuring that the support elements, all of which make contact with the wafer, actually retain the wafer, i.e. that they prevent the vibration of the wafer in the support point (and hence also in the vicinity thereof). This is achieved by bringing the support elements in frictional contact with the object carrier. The maximum frictional force of a support element should then be such that the forces which would be induced by the vibrations in the wafer in the vicinity of the relevant support element can be compensated by the frictional force. It has been found that for the known values of the friction coefficients between, for example metals, this condition can be readily satisfied in practical circumstances.
The support elements in a preferred embodiment of the invention are constructed as a cylindrical body which is arranged in a cylindrical cavity and is in frictional contact with the inner wall of the cylindrical cavity. Support points are thus obtained with a suitably defined location and the element providing the actual resilience (for example, a coil spring) can be firmly located in the cylindrical cavity so that it can exert in suitably reproducible spring force on the cylindrical body.
The cylindrical body in a further embodiment of the invention is provided with a protrusion which is eccentrically situated relative to the cylinder axis of the body and projects from the cylindrical cavity in a direction transversely of the support surface. In this embodiment it is simply achieved that the positioning of the object produces a frictional force which is proportional to the force exerted on said protrusion by the object. This is achieved by way of the eccentric arrangement of the protrusion so that when the cylindrical body is compressed, the body is laterally pressed against the inner wall of the cylindrical cavity. The resultant frictional force is proportional to the force acting on the protrusion in the direction of the cylinder axis.